The explosion of high speed communication devices and systems has led to the need for high speed integrated circuits. Circuits operating at speeds higher than one gigahertz are typically classified as microwave circuits and include microwave integrated circuits (MICs). MICs can include both passive and active elements. A typical passive element integrated circuit is a microstrip design, having a conductive ground back plane and a conductive pattern above the ground plane, resting on a dielectric. Passive microwave circuits are typically two ports devices having an input port and an output port. Active circuits include an active element, such as a gallium arsenide transistor, requiting a dc biasing voltage. Thus, in addition to input and output ports, active circuits require one or more dc biasing ports.
To ensure adequate MIC quality and functionality requires high frequency test setups. A typical test setup includes a test fixture to secure and provide contact to the MIC, and a testing system that connects to the test fixture. The high operating frequencies, high through-put, and small geometries of MICs gives rise to a number of testing problems. One common problem centers around how the test fixture makes contact with the MIC. Erratic and/or undefined signal or ground contact to the MIC can result in degraded or inconsistent test results. Thus, repeatable, nondestructive test fixture contact to both the signal and ground contacts of an MIC remains an important goal in the MIC industry.
Two port MIC designs will have an input port pad and an output port pad. Testing requires that a test fixture probe land on these pads to carry the signal input from the testing system to the device, and to carry the resulting signal output away from the device to the testing system. Three critical aspects arise from the test fixture probe-to-MIC pad contact. Such a contact must be in the proper location, have the proper contact force, and have sufficient contact definition between the probe and pad. Testing with a misaligned probe can result in the probe entirely missing the pad, or contacting an edge of the pad, creating an intermittent or high impedance contact, which in turn, creates erroneous testing results.
Test fixture probe force also plays a critical role in the testing of MICs. A probe arriving with too much force onto an input or output pad can tear through the pad, not only giving erroneous results, but also permanently damaging the MIC pad. Conversely, insufficient force can lead to intermittent contact, also resulting in erroneous test results.
Test fixture probe contact definition refers to the amount of surface area contacted by the probe. This aspect of testing MICs is important as, unless the probe contacts enough surface area of the circuit pad, a high contact resistance, or even intermittent test contact may result, again, adversely effect testing results.
Equally as important as the test fixture signal probe contacts, is the test fixture ground contact. The MIC ground is commonly a single conductive plate on the underside of the MIC package. By placing the MIC in a recessed area, improper ground contact, with relatively high inductance, can also occur. Presently existing test fixtures, wherein the circuit is simply dropped onto a parallel planar surface acting as ground, can create erroneous results if contact is made at different places on the back plane (i.e. if the surfaces are not perfectly flat). For example, on a first test, one edge of ground plane could be contact to the ground plate, while on a subsequent test the opposite edge could be contacted to the ground plate. In such an example, the varying current paths can result in varying test results.
Yet another testing problem arises from the fact that MICs are of microstrip design, and circuit test systems have coaxial inputs and outputs. The abrupt transition within test fixtures from a coaxial line to a microstrip can result in a relatively high mismatch in impedance, creating problems in calibration.
For all of the above reasons it is desirable to provide a microwave circuit test fixture that reduces the adverse of effects of abrupt coaxial-to-micro strip transitions. It is further desirable to provide a microwave test fixture that provides repeatable, nondestructive and accurate ground and signal contact, with proper contact location, geometry, force and definition.